It is well known that isobutylene may be reacted with methanol over an acidic catalyst to provide methyl tertiary butyl ether (MTBE) and isoamylenes may be reacted with methanol over an acidic catalyst to produce tertiary amyl methyl ether (TAME). The reaction is a useful preparation for these valuable gasoline octane enhancers and is typical of the reaction of the addition of lower alkanol to the more reactive tertiary alkenes, or iso-olefins, of the type R.sub.2 C.dbd.CH.sub.2 or R.sub.2 C.dbd.CHR under mild conditions to form the corresponding tertiary alkyl ethers. The feedstock for the etherification reaction may be taken from a variety of refinery process streams such as the unsaturated gas plant of a fluidized bed catalytic cracking operation containing mixed light olefins, preferably rich in isobutylene and isopentenes or isoamylene.
Generally, it is known that asymetrical C.sub.5 -C.sub.7 alkyl tertiary alkyl ethers are particularly useful as octane improvers for liquid fuels, especially gasoline. MTBE, ethyl t-butyl ether (ETBE), isopropyl t-butyl ether (IPTBE) and TAME are known to be high octane ethers. The article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The octane blending number of MTBE when 10% is added to a base fuel (R+O=91) is about 120. For a fuel with a low motor rating (M+O=83) octane, the blending value of MTBE at the 10% level is about 103. On the other hand, for an (R+O) of 95 octane fuel, the blending value of 10% MTBE is about 114. Increasing demand for high octane gasolines blended with high octane ethers as octane boosters and supplementary fuels has created a significant demand for these ethers, especially MTBE and TAME.
In these etherification processes, a problem of major importance is the separation of methanol from the etherification reaction product due to the proclivity of methanol to form a very dilute azeotropic mixture with hydrocarbons and the strong solubility of methanol in both water and hydrocarbons. Due largely to these factors, the cost associated with methanol separation and recycling in the etherification reaction represents approximately 30% of the cost of the total etherification process. While it would be useful from an equilibrium standpoint to use large excesses of methanol in etherification, subsequent separation problems have limited that process improvement. Currently, preparation of MTBE and TAME is carried out using C.sub.4 + hydrocarbon feedstock where methanol is present in the etherification step in about less than a three weight percent excess based on iso-olefins in the feed. This is effective in converting over ninety percent of isobutylene to MTBE, but isoamylene conversion is limited to about sixty-five percent under these conditions. Attempts to improve the conversion of isoamylene to TAME by manipulating the chemical equilibria with large excesses of methanol while maintaining high conversion of isobutylene to MTBE have proven disappointing, incurring heavy economic burdens on separation of the product.
Representative teachings in the prior art directed to the effort to improve the iso-olefin etherificaton process include U.S. Pat. No. 4,647,703 to Torck et al. which describes a multi-stage etherification process wherein effluent from the first stage is passed to a fractionator, a bottoms product containing ethers is withdrawn, and a top product containing unreacted light olefins and alcohol is passed to a second stage etherification reactor.
In U.S. Pat. No. 4,554,386 to Groeneveld et al. an iso-olefin etherification process is disclosed wherein multiple reactors are employed. An MTBE separation column is positioned after the first reactor.
In U.S. Pat. No. 4,324,924 to Torck et al. a multi-stage process is disclosed for preparing MTBE wherein effluent from the first stage is fractionated and the overhead is passed to a second stage for processing.
It is an object of the instant invention to provide a process for the production of MTBE and TAME that includes high conversion of isoamylene to TAME.